Membrane electrode assembly, production method for the same, and proton-exchange membrane fuel cell

ABSTRACT

The membrane electrode assembly of the present invention for the proton-exchange membrane fuel cell includes a polymer electrolyte membrane and an electrode catalyst layer, wherein at least a part of the polymer electrolyte membrane infiltrates into the electrode catalyst layer, and wherein the polymer electrolyte membrane is formed by polymerizing a composition containing at least a compound having proton conductivity and a compound having activity to an active energy ray, or a composition containing at least a compound having proton conductivity and activity to the active energy ray. The object of the present invention is to provide a membrane electrolyte assembly for realizing a high-output proton-exchange membrane fuel cell by improving a bonding state between the polymer electrolyte membrane and the electrode catalyst layer to reduce an internal resistance, and by providing a three-dimensional three-phase interface to increase reaction areas.

TECHNICAL FIELD

The present invention relates to a membrane electrode assembly for aproton-exchange membrane fuel cell, a production method for theassembly, and the proton-exchange membrane fuel cell using the assembly.

BACKGROUND ART

A proton-exchange membrane fuel cell uses a reducing agent such as purehydrogen or reformed hydrogen from methanol or fossil fuel as a fuel,and air or oxygen as an oxidizing agent. The proton-exchange membranefuel cell consists of: a membrane electrode assembly, which is anassembly of a polymer electrolyte membrane as an electrolyte and a gasdiffusion electrode including an electrode catalyst layer, serving as ahydrogen electrode (anode) and an oxygen electrode (cathode); and meansfor supplying a reducing agent such as pure hydrogen or methanol as afuel and air or oxygen as an oxidizing agent.

In a proton-exchange membrane fuel cell using hydrogen as a fuel, forexample, the following reactions (1) and (2) take place in a negativeelectrode and a positive electrode, respectively.Negative electrode: H₂→2H⁺+2e⁻  (1)Positive electrode: ½O₂+2H⁺+2e⁻→H₂O  (2)

Protons generated at the negative electrode pass through the polymerelectrolyte membrane and transfer to the positive electrode. If thepolymer electrolyte membrane and the electrodes are insufficientlybonded, protons hardly transfer at interfaces between the electrodes andthe polymer electrolyte membrane, thereby increasing its internalresistance.

Further, a three-phase interface where a catalytic reaction takes placeforms at a bonded interface between the polymer electrolyte and theelectrode. The areas of the three-phase interface vary depending on abonding state of the polymer electrolyte membrane and the gas diffusionelectrode including the electrode catalyst layer.

In the proton-exchange membrane fuel cell, a catalytic reactionpresumably takes place at the three-phase interface where all of thepolymer electrolyte, the electrode catalyst, and a reaction gas (orliquid) exist. Thus, one of important factors affecting an electricitygeneration performance of the proton-exchange membrane fuel cell is theareas of the three-phase interface of: pores serving as supply paths ofthe reaction gas; the solid polymer electrolyte having protonconductivity; and catalyst particles, at the interface between thepolymer electrolyte membrane and the electrode catalyst layers.

In order to improve the electricity generation performance of theproton-exchange membrane fuel cell, a catalytic reaction site must bethree-dimensional for increasing reaction sites. Further, the solidpolymer electrolyte must be provided inside the electrode catalystlayers for transferring the protons rapidly.

As an example of the conventional method of producing a membraneelectrode assembly, Japanese Patent Application Laid-Open No. H8-106915proposed a method of sandwiching a solid polymer electrolyte membranebetween gas diffusion electrodes including electrode catalyst layers,and hot pressing the whole, to thereby bond the polymer electrolytemembrane and the gas diffusion electrodes including the electrodecatalyst layers.

However, the membrane electrode assembly produced according to theconventional production method still has insufficient bonding atinterfaces between the polymer electrolyte membrane and the electrodecatalyst layers of the gas diffusion electrodes and has an insufficientthree-dimensional three-phase interface. Thus, the internal resistanceof the fuel cell increases and utilization of the catalyst decreases,whereby sufficient output characteristics of the proton-exchangemembrane fuel cell cannot be obtained.

Further, bonding through hot pressing forms substantially flat bondedinterfaces between the polymer electrolyte membrane and the electrodecatalyst layers of the gas diffusion electrodes. It cannot be said thatthe bonding strength is sufficient under the electricity generationenvironment, and the interfaces may be peeled in some cases. Thus, it isnecessary to improve the bonding strength between the polymerelectrolyte membrane and the electrode catalyst layers.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above-mentionedbackground art, and an object of the present invention is to provide: amembrane electrode assembly for realizing a high-output proton-exchangemembrane fuel cell by improving a bonding state between the polymerelectrolyte membrane and the electrode catalyst layer to reduce internalresistance, and forming a three-dimensional three-phase interface toincrease the reaction area; and a high-output proton-exchange membranefuel cell using the membrane electrode assembly.

Further, the present invention provides a method of producing a membraneelectrode assembly by which the above membrane electrode assembly can beeasily obtained.

That is, a membrane electrode assembly for a proton-exchange membranefuel cell according to the present invention provides includes at leasta polymer electrolyte membrane and an electrode catalyst layer, whereinat least a part of the polymer electrolyte membrane infiltrates into theelectrode catalyst layer, and wherein the polymer electrolyte membraneis formed by polymerizing a composition containing at least a compoundhaving proton conductivity and a compound having activity to an activeenergy ray, or a composition containing at least a compound havingproton conductivity and activity to the active energy ray.

A reinforcement member composed of an electrical insulator is preferablyprovided inside the polymer electrolyte membrane.

Further, the method of the present invention for producing a membraneelectrode assembly for a proton-exchange membrane fuel cell, theassembly including at least a polymer electrolyte membrane and anelectrode catalyst layer, at least a part of the polymer electrolytemembrane infiltrating into the electrode catalyst layer, comprises thesteps of: coating the electrode catalyst layer with a compositioncontaining at least a compound having proton conductivity and a compoundhaving activity to an active energy ray, or a composition containing acompound having proton conductivity and activity to the active energyray, to form a precursor layer of the polymer electrolyte membranecomposed of the composition, at least a part of the compositioninfiltrating into the electrode catalyst layer; and polymerizing thecomposition by irradiating the precursor layer with the active energyray, to form a polymer electrolyte membrane at least a part of whichinfiltrates into the electrode catalyst layer.

The electrode catalyst layer preferably has a thickness of 0.01 to 200μm; and an infiltration amount of the composition into the electrodecatalyst layer is preferably equal to or smaller than the thickness ofthe electrode catalyst layer.

The polymer electrolyte membrane is preferably provided with areinforcer of an electrical insulator inside the membrane.

Further, the present invention provides a proton-exchange membrane fuelcell employing the membrane electrode assembly.

According to the present invention, a membrane electrode assembly havinga polymer electrolyte membrane at least a part of which infiltrates intoan electrode catalyst layer can be formed by irradiating with an activeenergy ray a composition containing at least a compound having protonconductivity and a compound having activity to the active energy ray, ora composition containing at least a compound having proton conductivityand activity to the active energy ray. Thus, a bonding state between thepolymer electrolyte membrane and the electrode catalyst layer improvesto reduce its internal resistance, and a three-dimensional three-phaseinterface is provided to increase reaction areas, thereby providing ahigh-output membrane electrode assembly.

Further, the present invention can provide a production method for amembrane electrode assembly by which the membrane electrode assembly canbe easily obtained.

Further, the present invention can provide a high-output proton-exchangemembrane fuel cell employing the membrane electrolyte assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial schematic view showing a proton-exchange membranefuel cell of the present invention; and

FIG. 2 is a schematic view showing a bonded surface of an electrodecatalyst layer and a polymer electrolyte membrane of a membraneelectrode assembly of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail withreference to the drawings.

FIG. 1 is a partial schematic view showing an example of aproton-exchange membrane fuel cell of the present invention.

In FIG. 1, the proton-exchange membrane fuel cell of the presentinvention includes: a polymer electrolyte membrane 1; electrode catalystlayers 2 a and 2 b on both sides of the polymer electrolyte membrane 1;diffusion layers 3 a and 3 b on the outsides of the electrode catalystlayers 2 a and 2 b; and electrodes 4 a and 4 b serving as a collectorand a separator on the outsides of the diffusion layers 3 a and 3 b.

In the present invention, an assembly of the polymer electrolytemembrane 1, the electrode catalyst layers 2 a and 2 b, and the diffusionlayers 3 a and 3 b is referred to as “membrane electrode assembly”.Further, each of gas diffusion electrodes is an assembly of thediffusion layer and the electrode catalyst layer, that is, a pair 5 a ofthe electrode catalyst layer 2 a and the diffusion layer 3 a, and a pair5 b of the electrode catalyst layer 2 b and the diffusion layer 3 b.

FIG. 2 is a schematic view showing a bonded surface of the electrodecatalyst layer 2 and the polymer electrolyte membrane 1. The membraneelectrode assembly of the present invention has such a feature that apart of the polymer electrolyte membrane 1 infiltrates into theelectrode catalyst layer 2 to form an integrated structure as shown inFIG. 2. Reference numeral 6 denotes an infiltration portion where thepolymer electrolyte membrane 1 partly infiltrated into the electrodecatalyst layer 2. Reference numeral 7 represents conductive carbonsupporting a catalyst.

One or both of the electrode catalyst layers 2 a and 2 b includeelectrode catalysts containing conductive carbon. The electrode catalystlayer 2 a on a fuel electrode side, for example, is formed of anelectrode catalyst containing conductive carbon carrying at least aplatinum catalyst.

Platinum group metals such as rhodium, ruthenium, iridium, palladium,and osmium, or alloys of platinum and those metals may be used, insteadof the platinum catalyst. When methanol is used as a fuel, inparticular, an alloy of platinum and ruthenium is preferably used.

A catalyst used in the present invention is preferably carried on thesurface of conductive carbon. An average particle size of the carriedcatalyst is preferably small, specifically in a range of 1 to 10 nm. Anaverage particle size of less than 1 nm provides too high activity forcatalyst particles alone, leading to difficulties in handling. Anaverage particle size exceeding 10 nm reduces a surface area of thecatalyst to reduce reaction sites, which may deteriorate the activity.

Further, conductive carbon can be selected from the group consisting ofcarbon black, a carbon fiber, graphite, and a carbon nanotube. Anaverage particle size of conductive carbon is preferably in a range of 5to 1,000 nm, more preferably in a range of 10 to 100 nm. Further, aspecific surface area of conductive carbon is preferably relativelylarge for carrying the above-mentioned catalyst, and a BET specificsurface area thereof is preferably 50 to 3,000 m²/g, more preferably 100to 2,000 m²/g.

The electrode catalyst layer 2 b on an oxidizing agent electrode(cathode) side is formed of a similar electrode catalyst.

The diffusion layers 3 a and 3 b are coated with the electrode catalystalone or in combination with a paste prepared by mixing the electrodecatalyst with a binder, a polymer electrolyte, a water repellent,conductive carbon, and a solvent, and the coating is then dried.

The diffusion layers 3 a and 3 b serve to efficiently and uniformlyintroduce hydrogen, reformed hydrogen, methanol, or dimethyl ether as afuel and air or oxygen as an oxidizing agent into electrode catalystlayers, and the diffusion layers in contact with electrodes also serveto transfer electrons which contribute to a cell reaction. Generally, aconductive porous membrane is preferable as the diffusion layers, andfor example, carbon paper, carbon cloth, and a composite sheet of carbonand polytetrafluoroethylene can be used.

Surfaces and insides of the diffusion layers may be subjected to waterrepellent treatment with fluorine-based coating before use.

The diffusion layers preferably have a thickness of 0.1 to 500 μm. Thediffusion layer having a thickness of less than 0.1 μm showsinsufficient gas diffusion and water repellency. The diffusion layerhaving a thickness exceeding 500 μm undesirably increases its electricalresistance and ohmic potential loss. The diffusion layers morepreferably have a thickness of 1 to 300 μm.

The electrode catalyst layers are formed by coating on the surface andin the pores of the diffusion layers. The electrode catalyst layerspreferably have a thickness of 0.01 to 200 μm. A thickness of less than0.01 μm cannot provide an electrode catalyst layer having acatalyst-carrying amount which exhibits sufficient electricitygeneration performance. Further, the electrode catalyst layer having athickness exceeding 200 μm significantly reduces a gas diffusionproperty in the electrode catalyst layers while its electricalresistance increases. The electrode catalyst layers more preferably havea thickness of 0.1 to 100 μm.

A coating amount of a precious metal catalyst such as an alloy ofplatinum and ruthenium is preferably 0.01 to 10 mg/cm² (as calculated ina precious metal weight per area), more preferably 0.1 to 0.5 mg/cm². Acoating amount of less than 0.01 mg/cm² deteriorates the performance,and a coating amount exceeding 10 mg/cm² increases the cost.

Next, a polymer electrolyte membrane is formed by coating a surface ofthe electrode catalyst layer coated on the diffusion layer with acoating liquid composed of a composition containing at least a compoundhaving proton conductivity and a compound having activity to an activeenergy ray, or a composition containing at least a compound havingproton conductivity and activity to the active energy ray, and thencarrying out a polymerization reaction with active energy rays.

Hereinafter, the coating liquid which becomes a polymer electrolytemembrane with irradiation of the active energy ray and composed of acomposition containing at least a compound having proton conductivityand a compound having activity to the active energy ray, or acomposition containing at least a compound having proton conductivityand activity to the active energy ray will be simply referred to as“coating liquid”.

The compound having proton conductivity is preferably a compound havinga functional group such as a sulfonic group, a sulfinic group, acarboxylic group, a phosphonic group, a phosphoric group, a phosphinicgroup, and a boronic group. Specific examples thereof include: a mixtureof a polar polymer such as polystyrene sulfonic acid, polyvinyl sulfonicacid, polyaryl sulfonic acid, poly(meth)acrylic sulfonic acid,poly(meth)acrylic acid, poly(2-acrylamide-2-methylpropane sulfonicacid), polyacrylamide, polyethyleneimine, polyvinyl alcohol, orpolyethylene oxide and an inorganic acid such as sulfuric acid,phosphoric acid, or hydrochloric acid; a polymer obtained by introducinga sulfonic group or phosphoric group to a heat resistant polymer such aspolybenzimidazole or polyetheretherketone; and a perfluorocarbon-basedion exchange polymer represented by Nafion.

Further, the compound having activity to the active energy ray includesa monomer. Further, it may contain a crosslinking agent, an initiator,and the like.

The monomer includes a functional monomer or an oligomer having at leastone hetero atom. Specific examples of the monomer include:(meth)acrylates and di(meth)acrylates having oxyalkylene chains such asω-methyloligooxyethyl methacrylate; alkyl (meth)acrylates such as methylmethacrylate and n-butyl acrylate; (meth)acrylamide-based compounds suchas acrylamide, methacrylamide, N,N-dimethylacrylamide,N,N-dimethylmethacrylamide, acryloyl morpholine, methacrloyl morpholine,and N,N-dimethylaminopropyl(meth)acrylamide; N-vinylamide-basedcompounds such as N-vinylacetamide, and N-vinylformamide; alkyl vinylethers such as ethyl vinyl ether; and multifunctional (meth)acrylatessuch as trimethylolpropane tri(meth)acrylate, pentaerythritolpenta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

As a crosslinking agent, at least one multifunctional polymerizablecompound can be used by mixing as a copolymer component. Examples of acrosslinking multifunctional polymerizable compound capable of carry outcopolymerization include: diacrylates or dimethacrylates of polyalkyleneglycol having a molecular weight of 1,000 or less (such as oligoethyleneoxide, polyethylene oxide, oligopropylene oxide, and polypropyleneoxide); diacrylates or dimethacrylates of linear, branched, or cyclicalkylene glycol having 2 to 20 carbon atoms (such as ethylene glycol,propylene glycol, trimethylene glycol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol,1,10-decanediol, and cyclohexane-1,4-diol); multifunctional acrylate ormethacrylate compounds having a linear, branched, or cyclic polyvalentalcohol having three or more OH groups such as glycerin,trimethylolpropane, pentaerythritol, sorbitol, glucose, and mannitewherein two or more of the OH groups are substituted with an acryloyloxygroup or methacryloyloxy group (for example, trimethylolpropanetriacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTM),pentaerythritol triacrylate (PETA), pentaerythritol trimethacrylate(PETM), dipentaerythritol hexaacrylate (DPHA), and dipentaerythritolhexamethacrylate (DPHM)); multifunctional acrylate compounds having amolecular weight of 2,000 or less and having the above-mentionedpolyvalent alcohol wherein two or more of the OH groups are substitutedwith an acryloyloxy-oligo(or poly)ethylene oxy(or propylene oxy) group;multifunctional methacrylate compounds having a molecular weight of2,000 or less and having the polyvalent alcohol wherein two or more ofthe OH groups are substituted with a methacryloyloxy-oligo(orpoly)ethylene oxy(or propylene oxy) group; aromatic urethane acrylate(or methacrylate) compounds such as a reaction product of tolylenediisocyanate and hydroxyalkyl acrylate (or methacrylate) such ashydroxyethyl acrylate; aliphatic urethane acrylate (or methacrylate)compounds such as a reaction product of aliphatic diisocyanate such ashexamethylene diisocyanate and hydroxyalkyl acrylate (or methacrylate)such as hydroxyethyl methacrylate; divinyl compounds such asdivinylbenzene, divinyl ether, and divinyl sulfone; and diallylcompounds such as diallyl phthalate and diallyl carbonate.

Examples of the initiator include: radical thermal polymerizationinitiators such as azobisisobutyronitrile and benzoyl peroxide; radicalphoto polymerization initiators such as benzyl methyl ketal andbenzophenone; cationic polymerization catalysts such as protonic acids,e.g., CF₃COOH and Lewis acids, e.g., BF₃ and AlCl₃; and anionicpolymerization catalysts such as butyl lithium, sodium naphthalene, andlithium alkoxide.

The content of the compound having activity to the active energy ray inthe composition is 0.1 to 90 wt. %, preferably 1 to 80 wt. % withrespect to the compound having proton conductivity. A content of lessthan 0.1 wt. % may undesirably result in insufficient polymerization ofthe composition, and the content exceeding 90 wt. % may undesirablyreduce the proton conductivity of the electrolyte membrane.

Further, a compound having proton conductivity and activity to an activeenergy ray at the same time can be preferably used as well.

Examples of a compound having a sulfonic group include2-acrylamide-2-methylpropane sulfonic acid,2-methacrylamide-2-methylpropane sulfonic acid, sulfoethyl methacrylate,3-allyloxy-2-hydroxy-propanesulfonic acid, p-styrene sulfonic acid,allyl sulfonic acid, and vinyl sulfonic acid. Examples of a compoundhaving a carboxylic acid include acrylic acid, methacrylic acid,crotonic acid, maleic acid, fumaric acid, itaconic acid, and citraconicacid. A fluorine-based monomer or the like can also be used, ifrequired. The above compounds can be used alone, or as a mixture of aplurality thereof.

In particular, a (meth)acrylate derivative having a phosphate group on aside chain can be suitably used. This example is the trade name PhosmerM (acid phosphoxy ethyl methacrylate) commercially available fromUni-Chemical Co., Ltd.).

At least the compound having proton conductivity and the compound havingactivity to the active energy ray are mixed to thereby prepare thecoating liquid.

An appropriate solvent may be added for viscosity adjustment.

Further, a polymer and the like may be dissolved or dispersed in thecoating liquid as other additives.

Examples of the polymer include: polyethers such as polyethylene oxide,polypropylene oxide, polytetramethylene oxide, and polyhexamethyleneoxide; linear diols such as tetraethylene glycol, hexaethylene glycol,octaethylene glycol, and decaethylene glycol; poly(meth)acrylic acidssuch as poly(n-propyl (meth)acrylate), poly(isopropyl (meth)acrylate),poly(n-butyl (meth)acrylate), poly(isobutyl (meth)acrylate),poly(sec-butyl (meth)acrylate), poly(tert-butyl (meth)acrylate),poly(n-hexyl(meth)acrylate), poly(cyclohexyl (meth)acrylate),poly(n-octyl (meth)acrylate), poly(isooctyl (meth)acrylate),poly(2-ethylhexyl (meth)acrylate), poly(decyl (meth)acrylate),poly(lauryl (meth)acrylate), poly(isononyl (meth)acrylate),poly(isoboronyl (meth)acrylate), poly(benzyl (meth)acrylate), andpoly(stearyl (meth)acrylate); acrylamides such as polyacrylamide andpoly(N-alkylacrylamide); vinyl esters such as polyvinyl acetate,polyvinyl formate, polyvinyl propionate, polyvinyl butyrate, poly(vinyln-caproate), poly(vinyl isocaproate), poly(vinyl octanoate), poly(vinyllaurate), poly(vinyl palmitate), poly(vinyl stearate), poly(vinyltrimethylacetate), poly(vinyl chloroacetate), poly(vinyltrichloroacetate), poly(vinyl trifluoroacetate), poly(vinyl benzoate),and poly(vinyl pivalate); polyvinyl alcohol; acetal resins such aspolyvinyl butyral; polyolefins such as polyethylene, polypropylene, andpolyisobutylene; and fluorine resins such as polytetrafluoroethylene andpolyvinylidene fluoride.

The coating liquid has a viscosity of preferably 0.01 to 20 Pas, morepreferably 0.1 to 10 Pas. The viscosity of the coating liquid of lessthan 0.01 Pas provides too much coating liquid infiltrating into theelectrode catalyst layer and may clog the pores of the electrodecatalyst layers. The viscosity of the coating liquid exceeding 20 Pasdeteriorates fluidity and reduces the amount of the coating liquidimpregnated in the electrode catalyst layers.

The coating liquid prepared thus is coated on the electrode catalystlayer to be infiltrated into the electrode catalyst layers.

A coating method is not particularly limited. Specific examples thereofinclude: batch methods such as bar coating, spin coating, and screenprinting methods; and continuous methods such as preweighing andpostweighing methods. The postweighing method is a method of coatingwith an excess coating liquid, and then removing a part of the coatingliquid so as to provide a predetermined thickness. The preweighingmethod is a method of coating with an amount required to provide apredetermined thickness.

Examples of the postweighing method include air doctor coater, bladecoater, rod coater, knife coater, squeeze coater, impregnation coater,and comma coater methods. Examples of the preweighing method include diecoater, reverse roll coater, transfer roll coater, gravure coater,kiss-roll coater, cast coater, spraying coater, curtain coater, calendercoater, and extrusion coater methods. Screen printing and die coatingmethods are preferable for forming a uniform electrolyte membrane on theelectrode layer, and the continuous die coating method is preferable foreconomical reasons.

The infiltration amount of the coating liquid into the electrodecatalyst layers is preferably equal to or less than the thickness of theelectrode catalyst layers. The infiltration amount falls more preferablywithin the range of 1 to 30 μm for increasing a reaction area to providea high-output proton-exchange membrane fuel cell and for suppressing thecost. Further, the infiltration amount of the coating liquid into theelectrode catalyst layers can be adjusted to an arbitrary valuedepending on the viscosity and coating amount of the coating liquid.Further, the coating liquid may be infiltrated into the electrodecatalyst layer by bringing the electrode catalyst layer under a reducedpressure.

The coating thickness of the coating liquid on the electrode catalystlayer surface is 1 mm or less, preferably within the range of 5 to 500μm calculated as solid contents. The thickness of less than 5 μmprovides an electrolyte membrane having minute pinholes and cracksformed easily. The thickness exceeding 500 μm may increase its membraneresistance.

Further, a reinforcement member of an electrical insulator may beprovided on the electrode catalyst layer surface and then the coatingliquid is impregnated into it, or the reinforcement member impregnatedwith the coating liquid may be press-bonded on the surface of theelectrode catalyst layers, in order to reinforce the electrode catalystlayers and the polymer electrolyte membrane.

The reinforcement member having or not having hydrogen ion conductivitycan be used. Any forms of the reinforcement member including sheets,particulates, lines, fibers such as filaments and staples, wovenfabrics, and nonwoven fabrics can be used. Sheets, woven fabrics, andnonwoven fabrics are particularly preferable.

The reinforcement member is not particularly limited, and as itsmaterial, various resins can be used. Examples of such resins include:fluorine resins such as polytetrafluoroethylene and polyvinylidenefluoride; various polyamide resins such as 6,6-nylon; polyester resinssuch as polyethylene terephthalate; polyether resins such asdimethylphenylene oxide and polyetheretherketone; and copolymers ofα-olefins such as ethylene and propylene, alicyclic unsaturatedhydrocarbons such as norbornene, and conjugated dienes such as butadieneand isoprene. For example, polyethylene resins and polypropylene resins;and aliphatic hydrocarbon resins of elastomers such asethylene-propylene rubber, butadiene rubber, isoprene rubber, butylrubber and norbornene rubber, and hydrogenated elastomers thereof can beused. These resins may be used alone or in a mixture of two or morekinds thereof.

The reinforcement member may be subjected to a hydrophilic treatment bysuitable conventional means. Such a reinforcement member treated withthe hydrophilic treatment can be obtained by using a polymer having ahydrophilic group such as a sulfonic group, a phosphoric group, acarboxyl group, an amino group, an amide group, and a hydroxyl group ora mixture thereof as a raw material to form a film for the reinforcementmember. Also, the reinforcement member treated with the hydrophilictreatment can be obtained by forming a film of a polymer without such ahydrophilic group and then subjecting the film to, for example,sulfonation treatment.

Further, a polymer electrolyte membrane such as Nafion membrane(available from DuPont) separately prepared may also be used.

Further, an upper side of an electrode catalyst layer may be coated withthe coating liquid, and another electrode catalyst layer may bepress-bonded on the coated side, to thereby provide a structure of thecoating liquid sandwiched by the two electrode catalyst layers.Alternatively, the two electrode catalyst layers may be provided so asto sandwich a reinforcement member between these layers.

Next, the thus-produced stack of the electrode catalyst layers and thecoating liquid is irradiated with an active energy ray to simultaneouslycarry out formation of a polymer electrolyte membrane through apolymerization reaction of a composition in the coating liquid andbonding between the polymer electrolyte membrane and the electrodecatalyst layers.

Examples of the active energy ray that can be used include electronbeams, gamma rays, plasma, ultraviolet rays, and X-rays.

Electron beams, X-rays, and gamma rays are preferable because the rayreaches inside of the stack of the electrode catalyst layers and thecoating liquid and because irradiation equipment thereof costsrelatively low, thereby allowing reduction in process cost. Electronbeams and X-rays are particularly preferable because irradiation of therays is easy and its cost is low. Electron beams are most preferablebecause of high polymerization efficiency of monomers throughirradiation. Examples of an electron beam source include variouselectron beam accelerators such as a Cockcroft-Walton accelerator, a Vande Graaff accelerator, a resonance transformer accelerator, an insulatedcore transformer accelerator, a linear accelerator, a dynamitronaccelerator, and a high frequency accelerator.

An amount of electron beam irradiation is not particularly limited, butis set to preferably 100 Gy to 10 MGy, more preferably 1 kGy to 1 MGy,particularly preferably 10 kGy to 200 kGy. The amount of less than 100Gy results in insufficient polymerization of the composition in thecoating liquid. The amount exceeding 10 MGy may result in a fragilepolymer electrolyte membrane cross-linked three-dimensionally.

The acceleration voltage of the electron beams varies depending on athickness of the electrolyte membrane. The acceleration voltage for afilm having a thickness of about several to several tens μm ispreferably about 100 kV to 2 MV, and for a film having a thickness of100 μm or more is preferably about 500 kV to 10 MV. The acceleratingvoltage may be further increased when metals or the like are included ina mold to block the electron beams. A plurality of electron beams havingdifferent accelerating voltages may be irradiated. Further, theaccelerating voltage may be changed during electron beam irradiation.

Of energy rays, electron beams are particularly transmitted well throughorganic substances and therefore permeatted to the inside, therebyproviding an electrolyte membrane sufficiently bonded to the electrodecatalyst layer.

Further, if required, a heat treatment may be performed on the electrodecatalyst layer coated with the coating liquid during irradiation of anactive energy ray and/or the formed electrolyte membrane after. Further,after forming the polymer electrolyte membrane, the membrane may besubjected to treatment such as hot pressing in order to enhance bondingbetween the layers and the membrane.

A proton-exchange membrane fuel cell of the present invention isproduced by using a membrane electrolyte assembly produced as above, andstacking the polymer electrolyte, electrode catalyst layers, diffusionlayers and electrodes as shown in FIG. 1. A shape of the proton-exchangemembrane fuel cell is arbitrary. Further, a production method thereof isnot particularly limited, and a conventional method can be used.

EXAMPLE 1

Hereinafter, the present invention will be described by way of Examplesand Comparative Examples, but the present invention is not limitedthereto.

(Production of Electrode Catalyst Layer)

Carbon paper (TGP-H-30, available from Toray Industries, Ltd.) having athickness of 0.1 mm and subjected to water repellency treatment was usedas a diffusion layer. A paste prepared by sufficiently mixing 1 g ofcarbon carrying a 60 wt. % Pt—Ru catalyst (Pt:Ru=1:1, atomic ratio)(available from Tanaka Kikinzoku Kogyo K.K.) and 5 g of a 5 wt. % Nafionsolution (available from Sigma-Aldrich Co.) was used as an electrodecatalyst layer of an anode side (negative electrode). Carbon paper wascoated with the catalyst paste to a predetermined thickness using a barcoater, and then was dried under a reduced pressure at room temperature.

Carbon paper subjected to water repellency treatment was also used as adiffusion layer of a cathode side (positive electrode). A paste preparedby sufficiently mixing 1 g of carbon carrying a 60 wt. % Pt catalyst(available from Tanaka Kikinzoku Kogyo K.K.) and 5 g of a 5 wt. % Nafionsolution was used as an electrode catalyst layer of an anode side(negative electrode). Carbon paper was coated with the catalyst paste toa predetermined thickness using a bar coater, and then was dried under areduced pressure at room temperature.

(Coating Liquid)

Bis(methacryloyloxy) ethyl diphosphate (trade name P-2M, available fromUni-Chemical Co., Ltd.) was used.

(Reinforcement Member)

As Examples of the reinforcement member, a nylon mesh (mesh 508,available from Tokyo Screen Co., Ltd.) having a thickness of 70 μm, ascreen opening of 20 μm, and a wire diameter of 30 μm; a porous PTFEfilm (Microtex NTF, available from Nitto Denko Corporation) having athickness of 15 μm; and Nafion 115 (available from DuPont) having athickness of 130 μm were used.

(Production of Membrane Electrode Assembly)

The surface of an electrode catalyst layer was coated with the coatingliquid to a predetermined thickness calculated as solid contents using abar coater. Then, another electrode catalyst layer was press-bonded ontothe coated surface.

When the reinforcement member was used, the reinforcement member wascoated with the coating liquid to a predetermined thickness calculatedas solid contents using a bar coater. Then, the reinforcement member waspress-bonded with two electrode catalyst layers.

The stack of the electrode catalyst layers and the coating liquid wasirradiated with electron beams of 150 kGy at an accelerating voltage of150 kV using electron beam irradiation equipment (Eye electron beamEC250/15/180L, manufactured by Iwasaki Electric Co., Ltd.), to therebyobtain a membrane electrode assembly. The membrane electrode assemblywas produced as shown in Table 1. TABLE 1 Thickness of polymer Thicknesselectrolyte membrane of (as solid contents) (μm) electrode InsideOutside catalyst electrode electrode Rein- layer catalyst catalystforcement Example (μm) layer layer member Example 80 20 80 Nylon 1Example 10 0.5 200 None 2 Example 180 170 30 PTFE 3 Example 240 220 100Nafion 115 4(Note:)Thicknesses of the electrode catalyst layers and polymer electrolytemembrane as solid contents were measured through SEM observation of asection of the membrane electrode assembly after production thereof.

COMPARATIVE EXAMPLE 1

A nylon mesh having a thickness of 70 μm, a screen opening of 20 μm; anda wire diameter of 30 μm was coated with bis(methacryloyloxy) ethyldiphosphate using a bar coater. The mesh was irradiated with electronbeams of 100 kGy at an accelerating voltage of 150 kV using electronbeam irradiation equipment, to thereby obtain a polymer electrolytemembrane having a thickness of 100 μm. Carbon papers having electrodecatalyst layers (electrode catalyst layer thickness: 200 μm) for theanode and the cathode were arranged on both sides of the polymerelectrolyte membrane, and the whole was hot pressed at 90° C. and 9.8MPa for 10 minutes, to thereby obtain a membrane electrode assembly.

The membrane electrode assembly obtained in each of Examples andComparative Examples was sandwiched between separators, and fuel cellperformance was evaluated using a fuel cell evaluation apparatus(manufactured by Toyo Technical Corporation).

A 5 wt. % aqueous methanol solution was supplied to a fuel electrode(anode) side at 10 ml/min, and air under atmospheric pressure wassupplied to an oxidizing agent electrode side at 100 ml/min. Electricitywas generated while the whole cell was maintained at 75° C.

Table 2 shows a terminal voltage during discharge at a current densityof 0.25 A/cm². TABLE 2 Example Terminal voltage (V) Example 1 0.41Example 2 0.38 Example 3 0.36 Example 4 0.35 Comparative Example 1 0.29

The results of Table 2 show that voltage values between the terminals ofExamples are better than that of Comparative Example 1.

In Examples, the electrode catalyst layer is irradiated with activeenergy rays with the coating liquid infiltrated into the electrodecatalyst layer. Thus, a part of the polymer electrolyte membrane isformed inside the electrode catalyst layer to sufficiently form athree-phase interface, thereby presumably improving an outputperformance of the proton-exchange membrane fuel cell.

INDUSTRIAL APPLICABILITY

The membrane electrode assembly of the present invention in which atleast a part of the polymer electrolyte membrane is infiltrated into theelectrode catalyst layer can be formed by irradiating with an activeenergy ray a composition containing at least a compound having protonconductivity and a compound having activity to the active energy ray, ora composition containing at least a compound having proton conductivityand activity to the active energy ray. Thus, the bonding state betweenthe polymer electrolyte membrane and the electrode catalyst layerimproves to reduce its internal resistance, and the three-dimensionalthree-phase interface is provided to increase a reaction area.Therefore, the membrane electrode assembly of the present invention canbe employed for a high-output proton-exchange membrane fuel cell.

The production method for a membrane electrode assembly according to thepresent invention allows easy production of the above-mentioned membraneelectrode assembly.

This application claims priority from Japanese Patent Application No.2003-339798 filed Sep. 30, 2003, which is hereby incorporated byreference herein.

1. A membrane electrode assembly for a proton-exchange membrane fuelcell, comprising a polymer electrolyte membrane and an electrodecatalyst layer, wherein at least a part of the polymer electrolytemembrane infiltrates into the electrode catalyst layer, and wherein thepolymer electrolyte membrane is formed by polymerizing a compositioncontaining at least a compound having proton conductivity and a compoundhaving activity to an active energy ray, or a composition containing atleast a compound having proton conductivity and activity to the activeenergy ray.
 2. A membrane electrode assembly according to claim 1,wherein a reinforcement member composed of an electrical insulator isprovided inside the polymer electrolyte membrane.
 3. A production methodfor a membrane electrode assembly for a proton-exchange membrane fuelcell, the assembly comprising a polymer electrolyte membrane and anelectrode catalyst layer, at least a part of the polymer electrolytemembrane infiltrating into the electrode catalyst layer, the productionmethod comprising the steps of: coating the electrode catalyst layerwith a composition containing at least a compound having protonconductivity and a compound having activity to an active energy ray, ora composition containing at least a compound having proton conductivityand activity to the active energy ray, to form a precursor layer of thepolymer electrolyte membrane composed of the composition, at least apart of the composition infiltrating into the electrode catalyst layer;and polymerizing the composition by irradiating the precursor layer withthe active energy ray, to form a polymer electrolyte membrane at least apart of which infiltrates into the electrode catalyst layer.
 4. Aproduction method for a membrane electrode assembly according to claim3, wherein the electrode catalyst layer has a thickness of 0.01 to 200μm, and an infiltration amount of the composition into the electrodecatalyst layer is equal to or smaller than the thickness of theelectrode catalyst layer.
 5. A production method for a membraneelectrode assembly according to claim 3, wherein the composition iscoated after a reinforcement member composed of an electrical insulatoris provided on the electrode catalyst layer.
 6. A proton-exchangemembrane fuel cell comprising a membrane electrode assembly for aproton-exchange membrane fuel cell, the membrane electrode assemblycomprising a polymer electrolyte membrane and an electrode catalystlayer, wherein at least a part of the polymer electrolyte membraneinfiltrates into the electrode catalyst layer, and wherein the polymerelectrolyte membrane is formed by polymerizing a composition containingat least a compound having proton conductivity and a compound havingactivity to an active energy, or a composition containing at least acompound having proton conductivity and activity to the active energyray.